Wood is hydroscopic, which means that it shrinks or swells with changes in its moisture content (MC). Freshly cut green lumber generally shrinks as its moisture content falls over time. Before lumber is used for construction, it is therefore usually desireable to dry the wood to a moisture content that will be relatively stable during the service life of the lumber in a particular structure, to minimize changes in the size of the lumber after it is used in construction. Drying may also be a desireable step in the preparation of engineered wood composits or other wood products, or as preparatory step prior to impregantion of a wood product with compounds such as preservatives or fire retardants.
Drying lumber before placing it in service may have a number of advantages, including: providing dimensional stability; increasing strength and mechanical fastener holding power; reducing subsequent drying-related damage such as splitting and checking; decreasing susceptibility to biological stain, decay and insect attack; improving the capacity of wood to hold paint and other coatings; and reducing the weight of wood with a resulting decrease in shipping and handling costs.
There are a number of factors that affect the process of drying lumber, including the species of tree, lumber size, structural direction of wood, drying method, and method of lumber preparation. The interrelationship of these factors may be complex. For example, the moisture content of sapwood lumber is usually considerably higher than heartwood. However, this variation in initial moisture content may be offset in the drying process by the fact that sapwood has a higher permeability than heartwood. In green lumber, it is possible to detect moisture content variations between species, from pith to bark and at different height levels. For example, differences among spruce, pine, and alpine fir trees may include variability in the percentage of sapwood content between species, variability in the moisture content of the sapwood between species, variability in the heartwood moisture content between species and variability in moisture content single log from the sapwood-heartwood boundary to the pith region (dependednt in part on the tree height). Exemplary data gathered from pine and alpine fir trees at different height levels, and on various spruce and alpine fir logs of different diamenters, shows the sapwood contents as given in Tables 1a and 1b, respectively.
TABLE 1a Sapwood/Heartwood Ratio (%) for Spruce-Pine-Fir Trees at Varying Height Levels Tree Height (ft) Alpine Fir Spruce Pine 0 19.5 30.5 24.0 8 16.5 29.5 22.0 16 18.5 26.5 22.0 24 18.0 25.5 29.0 32 20.5 24.0 29.0 40 22.5 25.5 31.0 48 31.0 28.0 28.0 56 35.5 34.0 64 42.0 72 58.0 80 71.0 Average 20.9 28.1 35.5
TABLE 1b Sapwood Content of Various Randomly Selected Spruce and Alpine Fir Logs Spruce Alpine Fir Diameter Sapwood Content Diameter Sapwood Content (in) (%) (in) (%) 8.0 20.4 12.8 15.0 7.6 41.0 8.4 23.8 7.2 30.0 10.3 17.0 5.4 28.0 12.2 12.4 8.6 45.0 15.2 10.0 5.7 19.0 9.6 12.0 5.4 34.8 14.5 23.0 10.8 19.6 9.3 21.4 8.0 22.0 Average 8.2 27.7 11.4 15.0
A consequence of the wide variability in moisture content and wood drying properties is that drying processes that typically treat large volumes of lumber with a uniform process may give varying results for different parts of the treated lumber. For example, alpine fir dried in a conventional kiln using a typical drying schedule may produce lumber with a variable final moisture content, shown in Table 2, indicating that even if the average moisture content of the lumber was within a selected maximum limit (such as 19%), a large volume of the lumber may still have a moisture content over that limit. For example, in the exemplified data, when the average moisture content was down to 18.6%, it was estimated that about 48% of the lumber still had a moisture content over 19%, and about 78% of the lumber had a moisture content over 12%. This is demonstrative of inefficiencies in conventional drying processes. These inefficiencies may be particularly important in some applications, such as value-added manufacturing of composite wood products, where it is desired to obtain a relatively uniform dryness in each piece of lumber that is to be used for making the composite, so that the parts of the composite have a similar moisture content.
TABLE 2 Moisture Content (MC) of Alpine Fir Lumber at Various Stages of Drying in a Commercial Conventional Kiln Drying Average Approximate Quantity of Lumber Pieces ( Time (hr) MC (%) MC &gt; 12% MC &gt; 19% 32.5 32.6 95.1 86.2 44.5 23.8 88.1 68.4 55.0 18.6 77.6 48.2 74.5 10.6 38.6 4.0
Variations in wood structure and permeability, relative proportions of sapwood and heartwood, and differences in specific gravity, original moisture content and moisture distribution, and refractiveness of the wood all contribute to differences in the drying properties of different species. One of the causes of low permeability, as well as large variation in permeability within a species, is the presence of discontinuities in a particular piece of lumber, such as wet pockets or knots.
There are two main structural directions in wood, namely: longitudinal and transverse. The longitudinal direction corresponds to the direction along the stem or trunk of the tree. The long dimension of most cut lumber is along this longitudinal direction of the wood. The transverse direction is perpendicular to the longitudinal direction of the stem. There are two structurally distinct transverse directions in wood, namely: radial and tangential. The radial direction is parallel to the radius of the stem, passing from the bark through the pith perpendicularly to the annual growth rings of the tree. The tangential direction is perpendicular to the radial direction and tangent to the annual growth rings of the tree.
The width and thickness of most cut lumber is along a transverse direction of the wood, the transverse direction typically having a component that is radial and a component that is tangential. These structural directions in wood are important to the drying process because wood is in part composed of elongated water-carrying channels (some of which carry fluids other than water, such as sap, under physiological conditions), most of which are oriented in the longitudinal direction of the stem. The longitudinal orientation of these passageways dictates that lumber is an anisotropic material in which the rate of fluid flow is different in the transverse and longitudinal directions.
Moisture movement in lumber is typically much slower in the transverse direction compared to the longitudinal direction. It has for example been calculated that the diffusion coefficient in the longitudinal direction may be about six times as great as that in the transverse direction (Brown, H. P., A. J. Panshin and C. C. Forsaith. 1952. Textbook of Wood Technology, Vol. II. 1.sup.st Ed.). Although moisture movement may be proportionally much more rapid in the longitudinal direction, the usual dimensions of cut lumber dictate that moisture migration in the transverse direction may be more important in conventional drying processes. This can give rise to difficulties in drying thick pieces of lumber, which have relatively large transverse dimensions.
There are a number of known methods for drying lumber, including: air drying, which is a relatively slow process; kiln drying, which uses high temperatures and air circulation to increase the drying rate; radio frequency/vacuum drying, in which the wood is heated by radio frequency irradiation and subjected to vacuum; superheated steam/vacuum drying, in which the lumber is heated with superheated steam. There are drawbacks to some conventional drying methods. For example, in radio frequency drying of lumber with a large longitudinal dimension, internal burning of the wood may occur when portions of the wood reach a relatively low moisture content, while other portions of the wood remain at a relatively high moisture content.
A number of innovative methods have been suggested for improving conventional drying processes. For example, U.S. Pat. No. 5,075,131 to Hattori et al. discloses a method for treatment of wood that includes forming small holes in the surface of the wood to assist in the impregnation of the wood with a preservative and to facilitate drying of the wood. Such methods of introducing very small holes or incisions in the wood may be intended to minimize surface damage, and thereby preserve the aesthetic appearance of the wood, the dimensions of the holes do not readily permit the holes to be refilled, except perhaps with a surface coating. In some applications, the presence of many small holes in the surface of a piece of lumber may be aesthetically undesirable.
The appearance of finished lumber may be improved by removing defects such as knots and knotholes. A wide variety of methods are known for detecting and repairing naturally occuring defects in lumber. For example, U.S. Pat. Nos. 4,894,971 and 5,440,859 disclose methods of replacing defects such as knots with a shaped plug. Automated systems have been suggested for detecting defects such as knots, and using such information to grade lumber or to effect repairs. Examples of such systems are disclosed in U.S. Pat. No. 4,984,172 issued to Luminari in 1991, U.S. Pat. No. 5,412,220 issued to Moore in 1995 and U.S. Pat. No. 5,585,732 issued to Steele et al. in 1996.